An oscilloscope is used to acquire, analyze and display electronic signal waveforms. The oscilloscope samples electronic signals and plots their waveforms on a cathode ray tube (CRT) display screen in units of voltage versus time. Conventionally, voltage amplitude is plotted along the vertical, that is, the Y-axis, and time is plotted along the horizontal, that is, the X-axis. In the past decade, digital oscilloscopes have been developed. The basic scheme in digital oscilloscope operation is the sampling of a signal, followed by analog-to-digital conversion of the voltage values of the acquired samples. The digitized information is then placed in memory and used to create the display of the signal's waveform. Overall, two chief concerns are the speed and the precision with which the oscilloscope acquires and ultimately displays a signal waveform. The optimal situation is obtained where maximum precision is achieved in a minimum amount of time.
The precision of a digital oscilloscope is often expressed in terms of its risetime. Risetime is analytically defined as follows: risetime=0.35/bandwidth, where bandwidth is the range of signal frequencies, commonly beginning at dc, which the oscilloscope can acquire. Risetime is equivalently defined as the amount of time required for a voltage signal to rise from ten percent to ninety percent of its final voltage amplitude. Hence, the risetime of an oscilloscope is the risetime of the highest frequency signal that the oscilloscope can acquire. The fastest risetime oscilloscopes are the most precise.
The overall speed of a digital oscilloscope is a function of the speed of acquisition and display routines which the oscilloscope must execute in order to acquire and display a signal waveform. Faster execution of acquisition and display routines will minimize oscilloscope "deadtime", that is, the time spent waiting for routines to complete. During deadtime, data may be missed. Therefore, minimizing sample acquisition and display times will maximize overall speed.
A digital oscilloscope acquires a signal by sampling it. A signal is sampled in cycles known as acquisition-sweeps. A sweep occurs whenever the signal crosses a threshold level, known as a trigger. Given the trigger, acquisition-sweep occurs as a function of a sample clock, such as a 25 MHz crystal oscillator. Samples of the signal are acquired on the rising edge of the sample clock, that is, every 40 nanoseconds, for a 25 MHz clock. The number of samples acquired per acquisition-sweep is the value of the user-selected time-range divided by the period of the sample clock. The time-range is the width, measured in units of time, of the display screen window. For instance, the time width of the display screen may be a factor of nanoseconds(ns), microseconds(us), milliseconds(ms), and so on up to some maximum time-range setting. Thus, with a 40 ns sample clock period and a time-range setting of 200 ns, the number of samples per sweep would be 200/40=5 samples/sweep. In addition, the display screen window can be partitioned into a fixed number of discrete time units called time-buckets. The time width of each time-bucket varies directly with time-range. The time-buckets are mapped into a waveform buffer in memory. Each time-bucket, whether in memory or on the screen, holds a single sample point.
Generally, two types of sampling techniques are used in digital oscilloscopes: single-shot and repetitive sampling. With single-shot sampling, also known as real-time sampling, the signal waveform is acquired on only one acquisition sweep but many points on the waveform are sampled. Once it is acquired, the waveform is displayed. With repetitive sampling, on the other hand, the signal waveform is acquired on repetitive acquisition sweeps and typically fewer points are sampled per sweep. The advantage of single-shot sampling is that non-periodic, "one-time event" signals can be acquired. The advantage of repetitive sampling is that high frequency periodic signals can be acquired.
A repetitive sampling technique known as random repetitive sampling allows for the capture of higher frequency periodic signals than the sampling rate would otherwise permit, while providing pre-trigger information. In random repetitive sampling, when the input signal crosses a triggering threshold level, a precise measurement is made of the time between the trigger and the time of the next sample, that is, the beginning of the next period of the sample clock. This measurement is used to assign each sample a time coordinate relative to the trigger. This method of sampling is random because there is no correlation between the time of the sample and the time of the trigger signal, that is, the two events are asynchronous. This randomness allows for the capture of a higher frequency signal than the rate of the sample clock would otherwise allow. This method of sampling is repetitive because the process is repeated at narrow time-range settings until enough points have been collected to accurately reconstruct the waveform for display. This repetitiveness allows for the capture of very high frequency signals.
With very fast risetime oscilloscopes which employ random repetitive sampling, overall instrument speed is decreased, that is, deadtime is increased, through iterative executions of sample acquisition-sweep routines and waveform-display routines, particularly if the display routines execute following individual acquisition-sweeps. As a way of minimizing deadtime, display routines may be postponed until a certain percentage of the time-buckets for a given time-range setting are filled with sample-point data. For instance, given 1000 time-buckets, display may occur whenever eighty-five percent, that is, 850, of the time-buckets have been filled. Using the proper percentage, however, is crucial. For example, in the wider time-ranges, signal frequencies well within the instrument's bandwidth may be inaccurately reconstructed for display if the percentage is too low and thus insufficient points are collected. On the other hand, in the more narrow, faster, time-ranges, if the percentage is too high then more than enough points to reconstruct the waveform may be collected thus delaying display during collection of the extra points.